This invention relates to multilamp photoflash units having circuit means for causing a different lamp to be flashed upon each occurrence of a firing pulse produced in synchronism with the opening of a camera shutter.
Numerous multilamp photoflash arrangements with various types of sequencing circuits have been described in the prior art. Series and parallel-connected lamp arrays have been shown which are sequentially fired by mechanical switching means, simple electrical circuits, switching circuits using the randomly varied resistance characteristics of the lamps, arc gap arrangements, complex digital electronic switching circuits, light-sensitive switching means and heat-sensitive switching devices which involve melting, fusing or chemical reaction in response to the radiant energy output of an adjacently located flashlamp.
One currently marketed eight-lamp photoflash unit employing radiation switches is described in U.S. Pat. Nos. 3,894,226 and 4,017,728 and referred to as a flip flash. A ten-lamp version is described in U.S. Pat. Nos. 4,156,269 and 4,164,007. The unit comprises a planar array of high voltage flashlamps mounted on a printed circuit board with an array of respectively associated reflectors. Circuitry on the board includes a plurality of solid state switches that chemically change from a high to low resistance, so as to become electrically conducting after exposure to the radiant heat energy from an ignited flashlamp operatively associated therewith. The lamps of the array are arranged in two equal groups disposed in the upper and lower halves respectively of the rec- tangular-shaped circuit board. A pair of terminal contacts at the lower end of the unit is provided for activation of the upper group of lamps while a set of terminal contacts at the top of the unit is operatively associated with the lower group of lamps. The application of sucessive high-voltage pulses (e.g., 500 to 4,000 volts from, say, a piezoelectric source controlled by the shutter of a camera in which the array is inserted) to the terminal contacts at the lower end of the unit causes the lamps at the upper half of the array to be sequentially ignited. The array is then turned end for end and again inserted into the camera in order to flash the remaining group of lamps. In the flip flash unit, therefore, all of the lamp sequencing circuitry is self-contained in the removable photoflash unit, and the only function of the camera is to produce successive firing pulses in response to actuation of the camera shutter.
Another type of multilamp array currently on the market employs lamp-reflector units in linear rows facing in opposite directions, such as described in U.S. Pat. Nos. 3,598,984; 3,598,985; and 4,032,769 referred to as a flash bar. Typically, such linear photoflash lamp arrays comprises a total of ten lamps arranged in two parallel rows of five lamps each, the lamps and reflectors of one row being staggered relative to the lamps and reflectors of the other row. The five lamps facing in one direction are connected to a respective firing circuit disposed on one side of a printed circuit board having a plug-in tab, and the five lamps facing in the opposite direction are connected to a respective firing circuit on the opposite side of the printed circuit board. Each of the lamps has a pair of lead-in wires, one of which is connected to a common circuit run which leads to a common terminal on the plug-in tab, while the other lead-in wire of each of the lamps of a group of five facing in one direction are connected to respective conductor runs leading to a set of five selective terminals on the respective side of the circuit board tab, e.g., see the aforementioned U.S. Pat. No. 3,598,985. The circuitry of this prior art flash bar unit contains no switching elements; hence, as described, for example in U.S. Pat. Nos. 3,618,492 and 3,757,643, one-at-a-time sequencing of the five lamps facing in one direction is provided by sequential application of low voltage firing pulses across the associated common terminal and successive ones of the associated selective terminals. That is, when the linear flash bar array is plugged into a camera, each time the shutter is actuated, electronic switching circuitry in the camera successively applies firing pulses in sequence to the five selective terminals (and associated common terminal) on the side of the circuit board to which the five lamps facing the camera subject are connected. When the five lamps facing in one direction are expended, the flash unit must be removed from the camera, rotated 180 degrees, and then reinserted in the camera circuit so that the five unused lamps are connected to the camera switching circuitry.
In the interests of compactness, simplicity of operation, and increasing the number of flash illuminated photographs that may be taken in rapid succession, it is desirable to provide a multilamp photoflash array in which all of the lamps face in the same direction and only a single set of connector terminals is required, so that removal and reorientation of the flash unit is unnecessary. Further, it is particularly desirable to provide a photoflash lamp array which is compatible with existing camera circuitry. Such a photoflash unit is described in copending U.S. application Ser. No. 217,722, filed Dec. 18, 1980, now U.S. Pat. No. 4,371,914, and assigned to the present assignee. The unit comprises a plurality of 2n flashlamps attached to a printed circuit board and disposed in a linear array with the longitudinal axes of respective lamps aligned in parallel. Circuit patterns on the cicuit board include a plurality of n selective terminal means, each associated with a respective pair of the lamps and connected to one lead-in wire of each of the lamps of the associated pair, and a common terminal associated with all of the 2n lamps but directly connected to the other lead-in wire of one lamp of each pair. The circuit board has a substantially rectangular portion with n spaced apart extensions projecting vertically from a longer side thereof, the extensions being located behind alternate lamps of the linear array. Each extension contains portions of the circuit patterns spaced apart to form predetermined gaps, and a mass of switch material is disposed on each extension to bridge the gap between circuit patterns to thereby provide a solid state radiation switch on each extension behind and adjacent to a respective lamp for receiving radiant energy emitted by that lamp. In this manner, a respective normally open switch on each circuit board extension is coupled between the common terminal and a lead-in wire of the other lamp of each pair of lamps, so that the n normally open switches are coupled between the common circuit lead-in wire of associated pairs of the 2n lamps.
In operation, common and selective terminals of the unit are adapted for connection to a source of firing pulses, and each of the switches is responsive to flashing of the directly connected lamp of an associated pair so as to connect the other lamp of that associated pair to the common terminal means. The lamps have a filament-type ignition means, and the resistance value of each lamp filament lies within a predetermined range of a resistance values. In this manner, the firing circuit to which the lamp array terminal means are connected can perform a monitoring function by passing a current of limited predetermined maximum value through each unexpended flashlamp to derive alignment signals which function to align a flashlamp sequencing circuit to bypass inoperative lamps. Accordingly, if the photoflash unit comprises ten lamps operated from a common terminal and five selective terminals, a first firing pulse will cause ignition of the directly connected lamp of the first pair of lamps, whereupon the normally open radiation switch asociated with that pair of lamps will be converted from a high to low resistance so as to directly connect the second of the first pair of lamps between the first selective terminal means and the common terminal means. Thereafter the second firing pulse will bypass the expended first lamp and cause the second lamp to be ignited via the actuated radiation switch connected between that lamp and the common terminal means. The third firing pulse will then be applied to the second terminal means connected to the second pair of lamps to ignite the lamp directly connected to that second terminal means. This firing sequence proceeds through the remaining lamps and lamp pairs as described with respect to the first lamp pair. The printed circuit board extensions particularly facilitate this operational arrangement in a compact, low-cost photoflash unit.
It is an object of the present invention to improve the just described photoflash lamp switching arrangement in a manner that will provide increased product reliability. The state of the art of photoflash manufacturing can result in a small percentage of open filament lamps, low oxygen pressure lamps, and other defects that can prevent proper lamp flashing. In the switching circuit described in the preceding paragraph, a defective lamp at a switch position, (i.e., an odd-numbered lamp in the above description) would result in two lost flashes. However, if redundant switching is employed, as described hereinafter according to the present invention, only one lamp would be lost in the above example. Thus, the failed flashed due to defective lamp can be reduced by as much as 50% in such a photoflash array.